Endothelial cells form a single cell layer that lines all blood vessels and regulates exchanges between the blood stream and the surrounding tissues. New blood vessels develop from the walls of existing small vessels by the outgrowth of these endothelial cells which have the capacity to form hollow capillary tubes even when isolated in culture. In vivo, damaged tissues and some tumours attract a blood supply by secreting factors that stimulate nearby endothelial cells to construct new capillary sprouts. Tumours that fail to attract a blood supply are severely limited in their growth.
The process whereby new vessels originate as capillaries, which sprout from existing small vessels, is called angiogenesis. It can therefore be seen that angiogenesis plays a major role in normal tissue development and repair and in the progression of some pathological conditions.
Once the vascular system is fully developed, endothelial cells of blood vessels normally remain quiescent with no new vessel formation, with the exception of the formation of new blood vessels in natural wound healing. However, a deregulation of blood vessel growth and an abnormal increase in vessel density can occur in diseases or conditions such as tumourigenesis, diabetic retinopathy, psoriasis and inflammation. Therefore the ability to inhibit inappropriate or undesirable angiogenesis may be useful in the treatment of these diseases or conditions.